Isolated: British soldiers will carry on in Helmand as US troops leave AFP/Getty Images

British soldiers in Afghanistan face a surge in attacks by Taliban fighters seeking to exploit an imminent withdrawal of US marines in the region where they are based. Experienced US soldiers are to be replaced by a fledgling Afghan army corps despite the American belief that there isn't a single Afghan military unit in the replacement force which is able to act independently.

The withdrawal plan is revealed in a new report by the US Department of Defense published before this weekend's Nato summit in Chicago. It describes the plans as "the biggest challenge for the remainder of 2012".

Estimates using Nato data suggest it will take more than four years before Afghan National Security Forces will be able to fight unaided.

Despite this, the coming months will see a 60 per cent reduction of US marines in Regional Command Southwest, which includes Helmand Province. By October, there will be fewer than 7,000 remaining from a force of more than 18,000.

UK military experts warn that British forces staying in Helmand could face an onslaught from Taliban forces looking to exploit the situation. One former British service chief, who would not be named, warned: "Next summer could be a very risky one for our people in Helmand. Look at it from the Taliban point of view: they'll be looking at the numbers being drawn down, thinking, 'Let's do something that will rock a country by doing a lot of killing'... it might well be in our area."

And Colonel Richard Kemp, former commander of British forces in Helmand, said: "We can probably expect to see our troops engaged in heavier fighting than they have been in the past couple of years. No matter how many Afghan soldiers there are there, I don't believe that they will be able to replace the effectiveness of the US forces that are leaving."

He added: "They are not close to being able to take over from Western forces unaided, and I don't believe that they would be able to contain the insurgency unaided by 2014, which is the date we are due to leave."

The warning comes as world leaders gathered in Chicago for the biggest ever summit on Afghanistan, with some 60 countries and organisations attending. Pressure to reduce the billions spent on Afghanistan has seen plans for a long-term Afghan force of more than 350,000 downsized to one of around 230,000.

Nato will confirm tomorrow that a new "train, advise, assist" mission will replace combat operations after 2014, with Afghan forces set to take the lead by the middle of 2013. However, in a report seen by senior Nato officials before the summit, fewer than one in 10 army and police units is rated "independent with advisers", according to an IoS analysis of the latest assessments by Nato commanders. It will take more than four years before all units could function with the help of only military advisers.

A Nato official admitted "[financial] sustainability is a factor" and that the Afghan forces will "need help for some time to come".

Revealing the reality behind David Cameron's past assurances of a "conditions-based" withdrawal, Sir William Patey, Britain's former ambassador to Afghanistan, said: "It has always been conditions-based in the sense that the one condition was that we'd all have our combat troops out by 2014."

In a statement, the Foreign Office described the prospects of Afghan forces being ready to take over by 2014 as "achievable" but "challenging". It described Britain's role in Afghanistan as centring on "trade, diplomacy and military training" and "significant funding" for the Afghan troops.

Many thousands of soldiers will still be needed after 2014, according to Michael O'Hanlon, senior fellow at the Brookings Institution and a member of General David Petraeus's advisory board at the CIA. "I think we'll need 10,000 to 20,000 troops focused on intelligence, special operations, air power, logistics, and training, as well as continued economic and security assistance to Afghanistan of more than $5bn a year," he said.

The Afghan army will be able to hold only major cities and roads by 2014 and "parts of the countryside will be contested and/or in Taliban hands," he added.

A series of scandals in recent months has boosted the Taliban's support – outrage at the burning of korans by US troops and the massacre of 17 civilians shot dead by a US soldier ignited a series of so-called "green on blue" attacks, where Afghan soldiers attacked Nato troops.

The threat from within has resulted in British soldiers being told to sleep with their weapons. The Ministry of Defence policy is that "all troops engaged in embedded partnering have access to a personal weapon at all times".

And the Afghan government is deploying hundreds of intelligence officers into the army in an attempt to weed out rogue elements, according to a Nato official.

Amid fears of Taliban infiltration, Afghan agents are focusing on soldiers with relatives in Pakistan to see whether "families have come under pressure from insurgents", they said.

Pakistan stands in the way of success in Afghanistan, according to the US Department of Defense, which, in its latest progress report on Afghanistan, accuses Pakistan of "passive acceptance – and in some cases, provision – of insurgent safe havens". It describes the relationship with Pakistan as one of "pervasive mistrust" and claims Pakistani sanctuaries are "the most critical threat to the Isaf [International Security Assistance Force] campaign in Afghanistan."

Expert analysis

"We might see a situation such as Vietnam, where the Vietnamese weren't prepared to fight for a corrupt government."

Colonel Richard Kemp; Former commander of British forces in Helmand Province

"In Colombia, and El Salvador ... a small outside force conducted counterterrorism, and trained national and local forces."

Dr Seth Jones; Senior fellow at the Rand Corporation think tank

"The biggest threat to success is that the international community doesn't come up with the money to support the Afghans."

Sir William Patey; Former British ambassador to Afghanistan

"We'll be handing over a country that hasn't been cleared to an Afghan military unable to do clearance."